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IPv6 - (Global Unicast) Address

影片資源

IPv6 Fundamentals: Purpose and Format of GUA

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Subnetting IPv6 Addresses

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Discovering IPv6 with Wireshark (SF21VEU)

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IPV6 Security Assessment Tools (Jeff Carrell)

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簡介

在 IPv6 中,Global Unicast Address 的地位和 IPv4 中的 Public IP 一樣,用來作為在整個網際網路上的識別號碼。而目前為止所分配出去的 Global Unicast Address 都是 2000::/3 中的所有地址。所以看到一個 IPv6 位址的前 3 位元是 001,就會知道這是一個 Global Unicast Address。舉例來說,Facebook 是 2a03:2880:f10c:83:face:b00c:0:25de

IPv6 一般性的敘述定義在 RFC4291,而而 Global Unicast Address 的敘述定義在 RFC3587

格式:分成前後 64 位元

原先的 IPv4 中,一個 Class 的 IP 可以再分割成多個 subnet; 或著多個連續的 IP 位址可以藉由 CIDR,將小的 IP 網段合併成一個較大的網段。而不管是哪一種方法,兩者都是藉由 IP 的前綴來作為封包路由的依據。而這個前綴的長度,則可能會因為該 IP 所屬的 Class 不同,或是 CIDR 中合併的小網段數目的不同,而有不同長度。

在 IPv6 中,採取的是相反的作法。不像 IPv4 分成 A, B, C 等級而有不同的前綴,一個 IPv6 的位址一律都是以前 64 位元作為那個路由用的前綴:

|         n bits          | 64-n bits |       64 bits              |
+-------------------------+-----------+----------------------------+
| global routing prefix   | subnet ID |       interface ID         |
+-------------------------+-----------+----------------------------+

而這 64 個位元的前綴,底下又細分為不同的欄位,作為不同階層的網路供應商或網段的分類依據。多級 page table:First time?。每一級可能由不同的供應商分配(可以參考 Subnetting IPv6 Addresses 這個影片中的解釋)所以有可能會像下面這樣:

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  • The server hosting the image is unavailable
  • The image path is incorrect
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前 64 位元 - Network Prefix

Global Routing Prefix (48 位元)

在目前的狀況中,每一個 Global Unicast Address 前三個位元恰好都是 001, 也就是一定會是一個屬於 2000::/3 的位址。也就是說:目前的 Global Unicast Address 壹定會具有以下的形式:

   An example of the resulting format of global unicast address under
   the 2000::/3 prefix that is currently being delegated by the IANA and
   consistent with the recommendations in RFC 3177 is:

    | 3 |     45 bits         |  16 bits  |       64 bits              |
    +---+---------------------+-----------+----------------------------+
    |001|global routing prefix| subnet ID |       interface ID         |
    +---+---------------------+-----------+----------------------------+

雖然現況如此,但實際上根據 RFC3587,這件事情在日後不一定會繼續成立:

   RFC 2374 was the definition of addresses for Format Prefix 001
   (2000::/3) which is formally made historic by this document.  Even
   though currently only 2000::/3 is being delegated by the IANA,
   implementations should not make any assumptions about 2000::/3 being
   special.  In the future, the IANA might be directed to delegate
   currently unassigned portions of the IPv6 address space for the
   purpose of Global Unicast as well.

Subnet Mask (16 位元)

IANA 分配完前 48 位元的部分之後,整個 global routing prefix 中剩下的 16 位元則可以給使用者自行決定該怎麼規劃 subnet。

Interface ID (64 位元)